Valve devices comprising a flow control valve where the passage of the control valve can be controlled by a thermocouple in such a way that the power of the fluid flow arriving at a user may be controlled depending on the temperature, are prior art. Such valve devices, which are commonly called “thermostat”, are used in many different applications, in particular for hydraulic pressurising media, which flows via a thermocouple-controlled orifice to a corresponding user at a temperature-dependent flow rate. When supplying users the reliable function of which is the prerequisite for the operational reliability of an associated system, as is the case for example with a hydraulic fan drive for a cooling system, care must be taken that the control of the flow rate that drives the respective hydraulic motor takes place in accordance with a corresponding specified characteristic curve.